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Aromatic Compounds

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Significance of Aromatic Compounds

Aromatic compounds have played an indispensable role in improving the quality of our lives in the past and continue to play the same in present. Computer parts, DVDs, and linchpin components of automotive parts are made up of aromatic compounds. Drugs such as Aspirin and paracetamol which we are used for ages are aromatic compounds. Not only this, the drug used for the treatment of malaria, chloroquine is also an aromatic hetropolycyclic compound that is also being studied to treat novel Coronavirus (COVID - 19) as well. The contribution of aromatic compounds is huge in the development of mankind and they still have more potential to do the same. So, in this scenario, it becomes a must for all of us to have at least a basic understanding of aromatic compounds. 

 

What are Aromatic Compounds? 

The chemical compounds that contain conjugated planar ring systems with delocalized pi-electron clouds instead of discrete alternating single and double bonds are called aromatic compounds. They are also known as aromatics or arenes. The most common example of aromatic compounds is benzene. These are unsaturated compounds that are stable in nature. 

 

Arene and Aryl Groups

Students generally get confused between arene and aryl groups. Let us clear this doubt here only. Arene is a compound containing one or more benzene rings. While when we remove a hydrogen atom from an arene, the aryl group is formed. It means benzene is an arene, but phenyl is an aryl group. All arenes are aromatic compounds but it's not necessarily that all aromatic compounds are arenes. 

 

Aromaticity

All aromatic compounds show aromaticity. The term aromaticity is used to describe a property of a cyclic, planar molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. The word aromaticity comes from the word ‘aroma’ which means fragrance or odor. Since most of the aromatic compounds are derivatives of benzene and benzene gives a distinct odor so, the compounds were named aromatic compounds. Although presently many non – benzene aromatic compounds have been discovered which do not have any odor. The term aromatic was first used by August Wilhelm Hofmann in 1855. Compounds must fulfill the following four conditions to be an aromatic compound –

  • The molecule must be cyclic. Example – Benzene and pyrrole are aromatic in nature while acyclic compound C4H5NH2 is a non-aromatic compound. 

Structures are given below – (image)

  • Every atom in the cyclic ring must be conjugated. As it will provide the cyclic ring delocalized pi-electron system. Thus, we can say every atom in the cyclic ring must have an empty p orbital and must be capable of participating in resonance. 

Examples are given below – (image)

  • Huckel’s Rule of Aromaticity: All compounds must obey Huckel’s Rule i.e. molecule must have 4n+2 pi-electrons where n is an integer (i.e. n= 0, 1, 2, 3, 4…etc.). For example,  Benzene has 6 pi-electrons and (4×1)+2 = 6, thus it obeys Huckel’s Rule while cyclooctatetraene has 8 pi-electrons 4n+2 ≠ 8, thus it does not follow Huckel’s Rule. So, benzene is aromatic and cyclooctatetraene is a non-aromatic compound.

  • The molecule should be planar or flat. Those compounds which follow the above 4 rules of aromaticity are generally flat as in that condition they possess large enough potential energy. 

 

Examples of Aromatic Compounds 

All aromatic compounds are hydrocarbons. A few examples of aromatic hydrocarbons are listed below –

  • Benzene

  • Aniline

  • Aspirin

  • Paracetamol

  • Benzyl Alcohol

 

Benzene

Is the best example for representing aromatic compounds. Scientists struggled a lot to determine the structure of benzene. Because benzene is unreactive towards addition reactions being a highly unsaturated compound. Thus, it shows exceptional stability. The cyclohexatriene structure for benzene was 1st proposed by August Kekule in 1865. After Kekule, many other scientists also proposed the structure of benzene. The quantum mechanical origins of stability or aromaticity were 1st modeled by Huckel in 1931. 

 

Benzene is a colourless, clear, highly flammable, and volatile, liquid aromatic hydrocarbon. Benzene is found in crude oils, as a by-product of oil-refining processes, as a solvent, and in the synthesis of numerous chemicals. It is formed from both natural processes and human activities. Natural sources of benzene are forest fires, volcanos, crude oil, gasoline, and cigarette smoke. Benzene is also used to make other chemicals that are used to produce nylon, synthetic fibers, plastics, and resins. It is also used to make some lubricants, dyes, detergents, types of rubbers, drugs, and pesticides. 

 

Aniline

It is also an aromatic compound. Most of the aromatic compounds are benzene derivatives. Although not all aromatic compounds are benzene based. With an amine attached to a benzene ring, aniline is the prototypical aromatic amine. It was first obtained by the destructive distillation of indigo in the year 1826. It’s named after the indigo-yielding plant Indigofera anil. 

 

Aniline is prepared commercially by the catalytic hydrogenation of nitrobenzene or by the action of ammonia on chlorobenzene and possesses somewhat of an unpleasant smell, like that of a rotten fish. It is used in manufacturing precursors to polyurethane. It is also used to make antioxidants, dyes, rubber accelerators, photographic chemicals, pharmaceuticals, urethane foams, petroleum refining, explosives, fungicides, and herbicides, among others.

 

Aspirin

Aspirin, also known as acetylsalicylic acid, is a drug used to reduce pain, fever, and inflammation. It was discovered by chemist Charles Frederic Gerhardt in 1853. Phenol, an aromatic compound, is one of the raw materials used in making the drug.

 

Paracetamol

It is also a drug that is used to treat fever and pain. It was 1st made in 1877. Since then we are using this medicine for pain relief. It appears to be safe during pregnancy and breastfeeding.

 

Benzyl alcohol

It is an aromatic alcohol. It is a clear colorless liquid with a pleasant odor, and is slightly denser than water. Like all aromatic alcohols, benzyl alcohol is also a compound in which the -OH group is not directly attached to the benzene ring. Its role is that of a solvent, an antioxidant, a metabolite, and a fragrance. It is used as an anti-parasite medication. It is mainly used as a medicine to treat head lice in people. 

 

Properties of Aromatic Compounds

  • Aromatic compounds have extremely high resonance energy.

  • These are stable unsaturated compounds.

  • They are generally non – polar and immiscible with water.

  • They give a sooty yellow flame due to the high ratio of carbon to hydrogen.

  • Aromatic compounds undergo substitution reactions rather than addition reactions.

  • These have delocalized pi-electrons. 

  • They show a coplanar structure.

  • They are used as a solvent for non - polar compounds.

  • They show electrophilic aromatic substitution and nucleophilic aromatic substitution reactions. 

 

Difference Between Aromatic Hydrocarbons and Aliphatic Hydrocarbons 

Both aromatic hydrocarbons and aliphatic hydrocarbons are organic compounds composed of hydrogen and carbon but still, they are different from each other and show different properties. Some of the differences between the two are listed below –

 

S. No.

Aromatic Hydrocarbons

Aliphatic Hydrocarbons

1

These hydrocarbons contain carbon and hydrogen attached in a ring system with delocalized pi-electrons

These hydrocarbons contain carbon and hydrogen attached in straight chains, branched chains, or in non-aromatic ring forms

2.

They show aromaticity.

They do not show aromaticity.

3.

All aromatic compounds follow Huckel's rule.

It is not necessary that ll aliphatic hydrocarbons will follow Huckel's rule


4.

Most aromatic compounds have a pleasant odor.


They do not have a pleasant odor.

5.

In these compounds, the carbon to hydrogen ratio is low.

In these compounds, the carbon to hydrogen ratio is high.

6.

These compounds generally burn with sooty flame (yellow).

These compounds burn with a non-sooty flame.

7.

These are unsaturated compounds.

Aliphatic hydrocarbons can be saturated and unsaturated.

8.

They have delocalized pi-electrons. 


They do not have delocalized pi-electrons.

9.

These are conjugated compounds.

The majority of these compounds are not conjugated.

10.

Benzene, toluene, naphthalene, etc. are examples of these compounds.

Methane, ethane, propane, etc. are examples of these compounds.

 

This ends our coverage of aromatic compounds. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to recognize aromatic compounds through their structures. If you are looking for solutions to NCERT Textbook problems based on this topic, then log in to the Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests, and much more.

FAQs on Aromatic Compounds

1. What is Toluene and What are its Usages?

Toluene is a liquid aromatic compound that is composed of 8 hydrogen atoms, 7 carbon atoms. It is configured in 6 ring-shaped atoms with a single atom appendix. It is used to manufacture intermediates like toluene di-isocyanate that helps in the production of polyurethanes. Polyurethanes are used in building insulation, coatings for floors and furniture, foam in furniture, mattresses, car seats, etc. Toluene is also used in glues, paints, gasoline, and explosives.

2. What Product is the Aromatic Compound Benzene used in?

Found in crude oil, and as a major part of gasoline, Benzene is mostly used in making drugs, pesticides, plastics, synthetic fibers, rubber lubricants, dyes, detergents, and resins. Benzene and toluene are also used in products like gasoline, hair dye, acrylic spray paint, nail polish, paint thinners, plastic cement, cleaning products, and airplane glue. Most benzene in our environment comes from the use of petroleum products. For further information, log in to the Vedantu website or download Vedantu Learning App.

3. What are the Most Common uses of Alkanes like Methane, Butane, and Propane?

Oil and natural gas are the main sources of Alkanes like methane, butane, and propane, among others. While Propane is mainly used in gas burners and aerosol sprays, Butane is used in cigarette lighters. Methane, on the other hand, can be converted to usable energy. It is mainly used as fuel in stoves, water heaters, and clothes dryers. One can get exposed to it through chemicals at work, cooking on a gas stove, being at landfills, or confined spaces like manholes, septic tanks, and sewers.

4. What are Electrophilic Aromatic Substitution Reactions?

Electrophilic aromatic substitution reactions are organic reactions where an electrophile replaces an atom that is attached to an aromatic ring. These reactions generally replace a hydrogen atom belonging to a benzene ring with an electrophile. In an electrophilic aromatic substitution reaction, the aromaticity of the aromatic system is preserved. Usually, these reactions require the use of a catalyst in the form of aluminum trihalides. Electrophilic aromatic halogenation reactions of aromatic rings with iodine, chlorine, or bromine lead to the production of Aryl halides or haloarenes. Two examples of electrophilic aromatic substitution are the nitration and sulfonation of benzene. 

5. What is the Difference between Phenyl and Phenol?

Phenol is an aromatic hydrocarbon compound. Its chemical formula is C6H5OH. The chemical structure of the phenol molecule has a benzene ring and a hydroxyl group attached to it. Phenyl is derived from phenol. The main difference between phenyl and phenol is that phenyl has no hydroxyl group and phenol has a hydroxyl group (-OH). When the hydroxyl group is removed from the phenol molecule, one gets phenyl. Another difference between the two is that phenol is moderately soluble in water as it can form hydrogen bonds with water. Phenyl is hydrophobic and is not a stable molecule.